Conventionally, signals transmitted or received by an antenna pass through a band-pass filter so that the antenna can transmit or receive a signal of a desired frequency band only. In such an antenna with a built-in filter, as its block diagram is shown in FIG. 9, a band-pass filter 84 is connected to a radiation element 81 through a connector 82 and a connection cable 83. As the filter connected to the antenna, when a steep damping characteristic is demanded, a coaxial resonant-type mono-block structured filter which is distribution constant filter is used, and when miniaturization is emphasized, a multilayer structured filter which is lumped constant filter is used. In general, however, the coaxial resonant-type mono-block structured band-pass filter is used.
If the distribution constant filter is used as the filter, since a size and a configuration corresponding to a wavelength to be used is required, the size is increased. Alternatively, if the multilayer structure filter is used, electrodes on the layers are connected to each other through a through-hole. However, a signal transmission path is locally narrowed in the connection using the through-hole, which increases high-frequency resistance or inductance and comes to increase in transmission signal loss or decline in the damping characteristic. Furthermore, in order to cut unnecessary electrical coupling between a plurality of circuit elements for the filter or transmission paths provided on a plurality of layers or on a surface of a layer, or unnecessary radiation, it needs to be shielded. Although the structure can be shielded in the vertical direction by providing an earth conductor film on the ceramic sheet, the shielding is not effective in the surface direction of the multilayer structure (in the direction parallel to the ceramic sheet) because an electric wave is leaked laterally from a gap of the connecting parts of the through-hole.
Meanwhile, in recent years, although as a radio device for cellular such as a portable telephone, two frequency bands such as Advanced Mobile Phone Service (AMPS for 0.8 GHz band in American standard) and Personal Communication Service (PCS for 1.8 GHz band in American standard which corresponds to PHS in Japan) are used, for example, the antenna for those tends to employ a dual band so as to be used for either frequency band. In addition, a system which can detect the place of the portable telephone by Global Positioning System (GPS) of 1.5 GHz band is thought. In addition, the terminal for cellular and a personal computer or the like tend to be connected by short-distance radio by worldwide standard of Bluetooth (BT for 2.45 GHz band). Thus, there is a demand for the antenna which can transmit or receive the signal of any frequency band among them in the portable telephone or the like.
In this case, as FIG. 10 illustrates a case where the antennas are disposed in the portable telephone, it is thought that the antennas for cellular and GPS are disposed at upper portions of the portable telephone and the antenna for BT is disposed at the lower portion of it. However, since further miniaturization is demanded in the portable device, to provide the antennas at various places in the portable device is contrary to the demand for the miniaturization.
As described above, according to the conventional antenna with a built-in filter, since the antenna and the band pass filter are connected through the connector and connection cable, the device becomes large and loss is increased. In addition, since the radiation element and the filter separately function, it is necessary to match the impedance of the radiation element, which further increase its size. Therefore, in view of reduction of the transmission loss and miniaturization of the device and further in view of the fact that an optimum performance can be obtained without matching the antenna to the filter on the user side, the antenna is preferably integrated with the filter.
Meanwhile, as described above, although there is a demand that the antenna which can be used for the plurality of frequency bands is mounted on a small portable device, according to the various applications, the distribution constant filter such as coaxial resonant-type mono-block structured filter and the lumped constant filter such as multilayer structured filter may exist together. However, if such different type filters are manufactured at the same time, since the manufacturing steps are different, the cost is increased and a size is increased because of the mono-block structure.
Furthermore, according to the above-described band-pass filter, as the frequency band is increased, the shielding characteristic deteriorates, and there can not be provided a broadband band-pass filter having superior shielding characteristic.
In addition, as described above, if the antennas for the plural frequency bands have integrally the filter built-in, it means that the radiation elements for the plural frequency bands such as cellular and GPS or BT and the band-pass filter are arranged together at one place, they are interfered with each other or the signal of another frequency band could be mixed in. In other words, as for the dual band antenna for cellular, since both are not used at the same time, reciprocal interference does not matter so much. However, there is a case where the antennas for cellular, GPS and Bluetooth are used at the same time, which could cause the reciprocal interference and lower the antenna performance.
Especially, the band-pass filter passes spurious which is an integral multiple of the frequency of the signal to be passed so that a noise is likely to enter. Therefore, for example, if the signal for AMPS of 0.8 GHZ is transmitted or received, since the signal for GPA of 1.5 GHz and the signal for BT of 2.45 GHz are about twice and triple of the frequency of the signal of AMPS, they are likely to enter as the spurious.
The present invention was made so as to solve the above problems and it is an object of the present invention to provide an antenna with a built-in filter having a structure in which an antenna and a filter are integrated while the filter formed of multilayer structure is miniaturized, and signals are not interfered to each other even when signals of a plurality of frequency bands are transmitted or received.
It is another object of the present invention to provide an antenna with a built-in filter having a structure in which a filter characteristic does not deteriorate by the reciprocal interference with a simple structure even when signals of a plurality of frequency bands are transmitted or received.
It is a further object of the present invention to provide an antenna with a built-in filter having a concrete structure in which a configuration is small, surface mounting is possible, antenna for plurality of frequency bands is combined and reciprocal interference can be prevented.
It is a still another object of the present invention to provide an antenna with a built-in filter in which the filter prevents high-frequency resistance or inductance from increasing while the filter built in the antenna is made small.
It is still a further object of the present invention to provide a concrete structure for integrating antenna with a built-in filter for cellular, GPS (Global Positioning System), Bluetooth or the like.
It is still a further object of the present invention to provide a dielectric multilayer structure in which when an electric circuit is formed in a multilayer structure of a dielectric material, elements formed on each of different layers are connected electrically without generating loss in the high frequency, or a shield characteristic of the surface direction in the multilayer body can be improved.